Aircraft landing gears and structures have stringent performance requirements. They are subjected to severe loading, corrosion, adverse environmental conditions and have complex shapes which vary from thin to thick sections. AISI 4340 steel and 300M steels are widely used for high stress aircraft landing gears and structures. These steels are not corrosion resistant and require protective coatings. Plating involves expensive toxic materials which pollute the environment and create health risks. Corrosion causes rust, cracks, and breaks and requires frequent inspections. Corrosion can fail aircraft landing gears and structures.
Ferrium S53 alloy U.S. Pat. No. 7,160,399 was primarily developed to provide high strength corrosion resistant steel for aircraft landing gears and structures. It is a cobalt-rich carbide precipitation strengthened corrosion resistant steel alloy. Ferrium S53 has several limitations (see Carpenter Technology Inc., Technical Datasheet, Carpenter Ferrium S53, www.cartech.com).                1. Ferrium S53 has corrosion resistance “Restricted” in salt spray and sea water tests.        2. Ferrium S53 requires passivation in 50% nitric acid solution to increase its corrosion resistance        3. Ferrium S53 has a yield strength (YS) of 220 ksi.        4. Ferrium S53 has high charge material costs.        5. Ferrium S53 requires complex and costly normalizing, annealing and heat treating procedures.        
A low cost high strength martensitic stainless steel is disclosed in the published US patent application No 20090196784. The steel described in No 20090196784 has several disadvantages:                1. The claimed ductility and toughness of the steel in the published application are based on an 8% wt. concentration of chromium. A concentration of 8% wt. insufficient to pass the salt spray test.        2. Increasing the concentration of chromium above 8% wt. reduces the claimed ductility and toughness because of the low concentration of nickel (0.1 to 3.0% wt.). The low concentration of nickel is not enough to supply the required ductility with an elongation of more than 10%, a reduction of area of more than 30% and a toughness with impact toughness energy of more than 14 ft-lb, and a fracture toughness of more than 50 ksi×(square root over (in)) (hereinafter “ksi√in”).        3. Increasing the concentration of nickel above 3.0% wt. disturbs the balance between austenite and ferrite stabilizing elements of the low cost high strength martensitic stainless steel so an additional strong ferrite stabilizing element should be added; however a lack of an additional strong ferrite stabilizing element does not allow a stabilization of the balance.        4. Austenitizing temperature of the published martensitic stainless steel is only 1850 to 1900 F. The higher austenitizing temperature, 1925 to 2050 F of the present invention allows reducing the sizes of carbides and increasing the concentration of carbon in a solid solution. Higher concentration of carbon in the solid solution and smaller carbides supply the present invention higher ductility, toughness and strength compared to published martensitic stainless steel.        5. The published martensetic stainless steel does not have Tungsten (W).        6. The low cost high strength martensitic stainless steel has a low homogenized anneal temperature, 2100 to 2150 F that does not allow the conducting of fully homogenized distribution of the elements. The higher homogenized anneal temperature, 2200 to 2375 F allows the obtaining of homogenous microstructure of the steel of the present invention and as a result increasing mechanical properties.        
The limitations of Ferrium S53 and the disadvantages of the steel of the US patent application No 20090196784 are overcome with the present invention.